My
guess is this...
Most warriors use a sword and a 'shield' of some type. In manga it is often the scabbard for the sword. One to protect, one to defend.
To cut off one arm of a adversary who is not your equal in skill is to cut off their ability to protect while leaving them the ability to defend themselves.
It would not be killing them in cold blood or leaving them helpless, both dishonorable things to do to a weaker opponent, but rather giving them a second chance to go and reflect on their foolishness that caused them to lose one arm in the first place.
It is a very visible symbol to all that you unwisely chose to battle against someone who was your better and they spanked you for your impudence.
Perhaps I have read too much into it, but as I said this is only my guess...